According to the RAF website non-specific "dye" is mixed with diesel ...

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How the smoke is created

The basic vapour colour is white, produced by injecting diesel into the hot exhaust from the jet engine. This reaches temperatures of over 400 degrees centigrade and vaporises immediately. The blue and red colours are made by mixing dye with the diesel. The dye and diesel is stored in a specially-modified pod fitted to each of the aircraft. The pilot releases the liquid by pushing one of three buttons on the control column. During the display each aircraft can produce smoke for a maximum duration of seven minutes. This gives the pilot five minutes of white smoke, and one minute each of red and blue smoke. For this reason, a ‘smoke plot’ is worked out extremely carefully to ensure that no aircraft runs out of smoke before the end of the display...

The dye is not easily removed if it stains skin and clothing. In order to protect the technicians against spillage, the Dye Team wear special protective silver overalls, thick gloves and goggles.

http://www.raf.mod.uk/reds/behindthescenes/dyeteam.cfm [Links inactive - To make links active and clickable, login or click here to register]

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Smoke released from aircraft was originally based on a mixture of 10-15% dye, 60-65% trichloroethylene or tetrachloroethylene, and 25% diesel oil, injected into the exhaust gases of the aircraft engines. Most commonly, teams now use specifically prepared liquid dyes and only gas oil, light mineral oil or a food grade white oil without harmful chlorinated solvents.